scsi_debug: add multiple queue support

Add submit_queue parameter (minimum and default: 1; maximum: nr_cpu_ids)
that controls how many queues are built, each with their own lock and
in_use bit vector. Add statistics parameter which is default off.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>